Radcliffe, who gave birth to daughter Isla on Jan. 17, said she sustained a stress fracture at the base of her spine during the 27-hour labor.
"Starting back with it already weak, and then with the relaxing of the hormones through pregnancy, all the bones do not come together," Radcliffe said. "There is no easy answer to when I need to be back to have a chance in Osaka."
Radcliffe began training again in late February in Boulder, Colorado. But the 33-year-old Englishwoman said she may have returned too early and might skip the defense of her world marathon title in Osaka to concentrate on next year's Beijing Olympics.
"I'm not going to take huge risks to make sure that I will be 100 percent to be there," Radcliffe said in British newspapers Wednesday. "But I am not ruling it out. It depends on how much time I am going to have off."
Radcliffe said she would rest completely for three weeks.
Radcliffe could choose to run the 10,000 meters in Osaka. She would need a qualifying time by Aug. 13. The women's marathon is Sept. 2 and the 10,000 on Aug. 25.
But her priority is Beijing after she failed to finish the marathon in Athens in 2004.
"I do not see this year as my last year at a world championship," she said. "It is more as a stepping stone. Beijing is really important."
Radcliffe set the women's world record of 2 hours, 15 minutes, 25 seconds when she won the London Marathon on April 13, 2003.